Aluminum nitride substrate and production method

ABSTRACT

An aluminum nitride substrate comprises a planer sintered aluminum nitride body containing carbon including free carbon “a”; and a one-dimensional or two-dimensional electrode (electrostatic electrode  3 ) buried in the sintered aluminum nitride body; wherein the sintered aluminum nitride body comprises: a work mounting portion “A” for placing a work thereon; and a base layer “B” having the electrode (electrostatic electrode  3 ) buried therein; an average concentration of free carbon “a” in the work mounting portion “A” being different from an average concentration of free carbon “a” in the base layer “B”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an aluminum nitride substrate for asemiconductor production and/or inspection apparatus, and a productionmethod of the same.

2. Description of the Related Art

Aluminum nitride (AlN), which has a relatively high thermal conductivityabout 320 W/mK, is excellent in electric insulation property, mechanicalstrength, and bondability to metal conductors, so that sintered aluminumnitride bodies obtained by firing aluminum-nitride powder have beenknown as IC substrates or package materials. However, it actually isuneasy to obtain a sintered aluminum nitride body high of thermalconductivity, by firing aluminum nitride powder. Thus, methods forimproving thermal conductivity of aluminum nitride have been developed.

One method includes: before firing, densifying particles of AlNexcellent in covalent bondability and difficult to be sintered, so thatthe sintered AlN body is improved in thermal conductivity. For example,there is disclosed a method of densifying AlN particles by addition of asintering aid (yttria (Y₂O₃)) in SHO-60 Annual Conference Proceedings ofthe Ceramic Society of Japan (in 1985, pp. 517-518, by Shinozaki etal.).

Another method includes: adding a substance bondable to oxides in asintered AlN body; and removing oxides therefrom, thereby improving thethermal conductivity of sintered AlN body. It is supposed that, aftersuch oxidation, the sintered AlN body has resultant voids, whichobstruct transmission of phonons contributing to heat conduction, sothat the thermal conductivity is decreased.

For example, Japanese Patent Application Laid-Open Publication No.60-127267 discloses a method of adding a sintering aid (yttria (Y₂O₃)),and trapping AlN oxides. Further, Japanese Patent Application Laid-OpenPublication No. 61-146769 (counterpart of U.S. Pat. No. 4,578,364 toHuseby et al.) discloses a method of adding carbon for a reaction withoxide, and removing products of the reaction.

In turn, Japanese Patent Application Laid-Open Publication No. 948668discloses an AlN ceramic having a sintered AlN body improved in thermalconductivity and strength (page 5, and FIG. 1). Further, Japanese PatentApplication Laid-Open Publication No. 2001-223256, addressed to asintered body of carbon-containing ceramic constituting a ceramicsubstrate for semiconductor production or inspection (page 8, and FIG.1), discloses rendering the concentration of carbon uneven, so that anelectrode pattern is prevented from being seen through the ceramicsubstrate, while adjusting the volume resistivity of ceramic substrate.

SUMMARY OF THE INVENTION

The sintered AlN body, if improved in thermal conductivity by adding anamount of particles of carbon (hereafter sometimes simply called“carbon”) into AlN powder in a conventional manner, tends to have anirregular-colored appearance, suffering from varieties of productquality. Particularly, adoption of AlN powder including excessive carbonadded thereto leads to a complicated firing so that crystal grains in asintered AlN body tend to be separated, thereby deteriorating thequality of a ceramic substrate. Further, when the excessively addedcarbon is partially left unreacted in the sintered AlN body,densification of the sintered AlN body is obstructed, to cause adeteriorated thermal conductivity of the sintered AlN body. In thisrespect, although the Japanese Patent Publication No. 60-127267 hasadded carbon in an amount to stoichiometrically react with oxygen(oxides) contained in the AlN powder, mere optimization of an amount ofcarbon fails to obtain a sintered AlN body having a high thermalconductivity.

Further, although the Japanese Patent Application Laid-Open PublicationNo. 2001-223256 has achieved the non-uniform carbon concentration in theceramic substrate, it is impossible to control the existence form ofcarbon so far, so that carbon possibly present on a grain boundary ofthe sintered AlN body tends to cause grain separation, thereby resultingin a deteriorated quality.

The invention has been made in view of the foregoing points, and it istherefore an object of the invention to provide a product whichsatisfies properties required by respective portions.

To achieve the object, properties required by respective portions areimproved by providing a variation in an average concentration of freecarbon on a sintered aluminum nitride body.

Namely, according to a first aspect of the invention, there is providedan aluminum nitride substrate, which comprises:

-   -   a planer sintered aluminum nitride body containing carbon        including free carbon; and    -   a one-dimensional or two-dimensional electrode buried in the        sintered aluminum nitride body;    -   wherein the sintered aluminum nitride body comprises: a work        mounting portion for placing a work thereon; and a base layer        having the electrode buried therein; an average concentration of        free carbon in the work mounting portion being different from an        average concentration of free carbon in the base layer.

Further, according to a second aspect of the invention, there isprovided a production method of an aluminum nitride substrate, whichcomprises the steps of:

-   -   using at least two types of aluminum nitride powders having        different carbon concentrations of 600 ppm or lower,        respectively, and burying an electrode within the powders and        molding them into an aluminum nitride mold in a structure        comprising at least two layers having different carbon        concentrations; and    -   firing the obtained aluminum nitride mold at 1,700° C. to 1,950°        C., into an aluminum nitride substrate comprising a work        mounting portion for placing a work thereon, and a base layer        having the electrode buried therein, an average concentration of        free carbon in the work mounting portion being different from an        average concentration of free carbon in the base layer.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The above and further objects, features, and advantages of the inventionwill appear more fully from the detailed description of the preferredembodiments, when the same is read in conjunction with the accompanyingdrawings, in which:

FIG. 1A is a cross-sectional view of an electrostatic chuck heaterprovided with an aluminum nitride substrate according to an embodimentof the invention, and FIG. 1B is an enlarged cross-sectional view of ametal coil;

FIG. 2 is a structural view of aluminum nitride forming the aluminumnitride substrate according to the embodiment;

FIG. 3 is a process chart of a production method of an aluminum nitridesubstrate according to the embodiment;

FIG. 4 is another process chart of a production method of an aluminumnitride substrate according to the embodiment;

FIG. SA is a cross-sectional view of a single electrode type ofelectrostatic chuck provided with an aluminum nitride substrateaccording to another embodiment of the invention, and FIG. 5B is aradial cross-sectional view of an electrostatic electrode portion of thechuck;

FIG. 6A is a cross-sectional view of a dual electrode type ofelectrostatic chuck provided with an aluminum nitride substrateaccording to still another embodiment of the invention, FIG. 6B is anaxial cross-sectional view of an electrostatic electrode portion of thechuck, and FIG. 6C is a radial cross-sectional view of the electrostaticelectrode portion;

FIG. 7A is a cross-sectional view of a heater provided with an aluminumnitride substrate according to yet another embodiment of the invention,and FIG. 7B is an axial cross-sectional view of a metal foil portion asa heater-aimed electrode; and

FIG. 8A is a cross-sectional view of a heater provided with an aluminumnitride substrate according to a still further embodiment of theinvention, and FIG. 8B is an axial cross-sectional view of a metal coilportion as a heater-aimed electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will be described into details aluminum nitride substrates andproduction methods thereof according to preferred embodiments of theinvention, with reference to the accompanying drawings.

FIG. 1A is a longitudinal cross-sectional view of an electrostatic chuckheater 2 adopting an aluminum nitride substrate 1 according to anembodiment of the invention. The aluminum nitride substrate 1 includedin the electrostatic chuck heater 2 is in a disk shape, and has a linearelectrostatic electrode 3 (or RF electrode) buried within an upper halfof the substrate, and a metal coil 4 buried in the substrate under theelectrostatic electrode 3. Applied to the metal coil 4 is an AC voltageby an AC power source 5. Placed on an upper surface of the aluminumnitride substrate 1 is a wafer 6, and the wafer 6 and the electrostaticelectrode 3 are connected to a cathode and an anode of a DC power source7, respectively. Thus, the wafer 6 and electrostatic electrode 3 areattracted to each other by coulomb forces acting therebetween, and alower surface of the wafer 6 and an upper surface of the aluminumnitride substrate 1 are attached to each other by Johnson-Rahbeck forcesacting therebetween, so that the wafer 6 is fixed to the upper surfaceof the aluminum nitride substrate 1.

The aluminum nitride substrate 1 is made of a sintered aluminum nitridebody containing carbon therein, and contains carbon, particularly, freecarbon which is segregated mainly at a crystal grain boundary. FIG. 2 isa structural view of the sintered aluminum nitride body, in which freecarbon “a” is present mainly at a crystal grain boundary of aluminumnitride particles “x”, and can be distinguished from solid solute carbon“b” dissolved in the crystal grains in a solid solution state. In FIG.1A, it is desirable that an average concentration of free carbon on awork mounting portion “A” of the sintered aluminum nitride body forplacing a work thereon, is equal to or higher than averageconcentrations of free carbon present between electrode segments and ina base layer “B” having the electrode buried therein. Note thatreference character B1 here designates a region of the base layer “B”under the electrode. FIG. 1B is an enlarged cross-sectional view of themetal coil in the electrostatic chuck heater 2 in FIG. 1A, and referencenumeral B2 designates a region between the electrode segments. Further,it is desirable that an average concentration of all carbon per unitvolume of the work mounting portion “A” is made higher or lower than anaverage concentration of all carbon per unit volume of the base layer“B” (region “B1” under the electrode and regions “B2” between theelectrode segments).

As average concentrations of solid solute carbon are increased todecrease average concentrations of free carbon, decrease of volumeresistance at high temperatures can be restricted, leakage current canbe decreased, and separation of crystal grains can be decreased innumber. Contrary, as average concentrations of solid solute carbon aredecreased to increase average concentrations of free carbon, thermalconductivity is increased. This makes it possible to provide productswhich satisfy properties required for respective portions of a ceramicsubstrate, by varying a distribution of average concentration of solidsolute carbon or free carbon on a sintered aluminum nitride body. It isdesirable that an average concentration of free carbon within a sinteredaluminum nitride body is 500 ppm or lower. This is because, higheraverage concentrations of free carbon lead to higher thermalconductivities, but average concentrations of free carbon exceeding 500ppm lead to rather complicated firing. Particularly, averageconcentrations of free carbon are preferably within a range of 10 ppm to200 ppm. Further, since average concentrations of all carbon in asintered aluminum nitride body exceeding 600 ppm lead to complicatedfiring, it is desirable that the average concentration of all carbon ispreferably 600 ppm or lower, particularly within a range of 300 ppm to550 ppm.

The electrostatic electrode 3 may be made of an electric conductor in alinear or planar shape, without limited thereto. Usable as theelectrostatic electrode 3 are a metal bulk body, a printed body, andelectrodes in meshed shape, foil-like shape, punching-metal shape, andelectroconductive paste state, respectively.

There will be explained a production method of the aluminum nitridesubstrate according to the embodiment of the invention, with referenceto FIG. 3.

Firstly, as shown in FIG. 3(a), aluminum nitride (AlN) powders 8 a, 8 band carbon powders 9 a, 9 b at carbon concentrations of 600 ppm orlower, respectively, are loaded into vessels 10 a, 10 b, to prepare twotypes of starting materials having different carbon concentrations,respectively.

Here, the aluminum nitride powders 8 a, 8 b are provided by producingaluminum nitride (AN) powders by the following reactions while adoptinga reductive nitriding method or direct nitriding method:Reductive nitriding method: Al₂O₃+3C+N₂→2AlN+3CODirect nitriding method: Al(C₂H₅)₃+NH₃→AlN+3C₂H₅, 2Al+N₂→2AlN

Meanwhile, usable as the carbon powders 9 a, 9 b are: powders of carbonsuch as carbon black, graphite; carbon-containing resins, which arespattering organic resins comprising powders of organic resin (such asphenol resin); and intermediate products of aluminum nitride havinghigher carbon concentrations, to be produced in the course of thereductive nitriding method. Concentrations of the carbon powders 9 a, 9b to be added to the aluminum nitride powders 8 a, 8 b are preferably600 ppm or lower, because concentrations exceeding 600 ppm make itimpossible to keep average concentrations of carbon within a sinteredaluminum nitride body after firing, within a preset range.Concentrations of the carbon powders 9 a, 9 b to be contained in thealuminum nitride powders 8 a, 8 b are preferably within a range of 260ppm to 450 ppm.

Next, as shown in FIG. 3(b), the aluminum nitride (AlN) powders 8 a, 8 band the carbon powders 9 a, 9 b are mixed with each other within thevessels 10 a, 10 b to obtain starting material powders, respectively.Note that the aluminum nitride powders 8 a, 8 b and carbon powders 9 a,9 b may be mixed with each other, respectively, by wet mixing whichutilizes an organic solvent, or dry mixing (such as ball milling,vibration milling, or dry bag mixing).

Thereafter, the two types of aluminum nitride powders containing carbonadded thereto are used to load an aluminum nitride powder, anotheraluminum nitride powder including a heater-aimed electrode buriedtherein, and still another aluminum nitride powder including anelectrostatic electrode buried therein, into a mold (not shown) in athree-layered manner, and then these powders are molded. This yields athree-layered aluminum nitride mold 16 formed of an aluminum nitridelayer 11, an aluminum nitride layer 13 over it and having a heater-aimedelectrode 12 buried therein, and an aluminum nitride layer 15 over itand having an electrostatic electrode 14 buried therein, as shown inFIG. 3(c) (molding step).

After molding, the aluminum nitride mold 16 is placed in a hot pressingapparatus 17 as shown in FIG. 3(d), and fired therein by hot pressing.Firing conditions preferably include: a firing temperature of 1,700° C.to 1,950° C.; a highest temperature keeping time of 0.5 hr to 100 hr, apressure of 50 kg/cm² to 250 kg/cm²; a temperature elevation rate of 10°C/hr to 120° C./hr; and a degree of vacuum of 1.3×10⁻¹ Pa to 133.3 Pa.The reason why the firing temperature is set at 1,700° C. to 1,950° C.is that, firing temperatures below 1,700° C. tend to complicatedissolution of carbon into a solid solution state, and contrary, firingtemperatures exceeding 1,950° C. cause color irregularities toconsiderably deteriorate a commercial value of a product. Further,firing in a high temperature range allows promotion of dissolution ofcarbon. The reason why the highest temperature keeping time at firing isset within the range of 0.5 hr to 100 hr, is that keeping times shorterthan 0.5 hr lead to insufficient amounts of solid solute carbon, andcontrary, keeping times exceeding 100 hr lead to prolonged producingtimes. Note that further prolonged highest temperature keeping times atabout 1,950° C. allow dissolution of carbon more. Pressures arepreferably within a range of 50 kg/cm^(2 to) 250 kg/cm². This isbecause, even higher pressures exceeding 250 kg/cm² fail to obtaineffects commensurating with such higher pressures, and contrary,pressures below 50 kg/cm² lead to deteriorated sintering abilities. Notethat although firing with application of pressures about 250 kg/cm²allows promotion of dissolution of carbon into a solid solution state,pressures are preferably varied depending on firing temperatures. Forexample, firing at higher firing temperatures and higher pressuresallows promotion of dissolution of carbon into a solid solution state,and contrary, firing at lower firing temperatures and higher pressuresallows an increased average concentration of carbon contained in analuminum nitride substrate. Further, temperature elevation rates arepreferably within a range of 10° C./hr to 120° C./hr. This is because,temperature elevation rates slower than 10° C./hr lead to smallerdifferences between average concentrations of carbon at inner and outerportions of an aluminum nitride substrate as a sintered body,respectively, and contrary, temperature elevation rates exceeding 120°C./hr result in cracking and/or deformation of the sintered body. Notethat larger temperature elevation rates allow increased differencesbetween average concentrations of carbon as mentioned above. Further,degrees of vacuum at firing are preferably set at 1.3×10⁻¹ Pa to 133.3Pa. This is because, degrees of vacuum lower than 1.3×10⁻¹ Pa lead tohigher average concentrations of carbon at an outer portion of analuminum nitride substrate, and degrees of vacuum exceeding 133.3 Palead to excessively smaller average concentrations of carbon at an outerportion of an aluminum nitride substrate. Further, setting an atmosphereupon firing at a degree of vacuum of about 133.3 Pa allows smalleraverage concentrations of carbon at an outer portion of an aluminumnitride substrate.

Note that the firing method is not limited to the above-mentioned hotpressing method, and other methods may be adopted. In the latter case,there is allowed a difference between amounts of solid solute carbon (orfree carbon amounts) at inner and outer portions of an aluminum nitridesubstrate, respectively, by appropriately combining a firingtemperature, a keeping time, a pressure, a temperature elevation rate,and a degree of vacuum, at firing.

Note that a metal coil or a metal mesh is placed within a molded bodyupon firing, depending on intended properties thereof. In this way, itis possible to determine a cut-out position of a product by utilizingdifferences in amounts of solid solute carbon and free carbon on asintered body, while placing the metal coil and metal mesh within themolding.

As shown in FIG. 3(e), yielded after firing is an aluminum nitridesubstrate 18 formed of: an aluminum nitride layer 19 comprising asintered aluminum nitride body having a higher average concentration offree carbon; an aluminum nitride layer 20 over the aluminum nitridelayer 19, comprising a sintered aluminum nitride body having a loweraverage concentration of free carbon(i.e., higher average concentrationof solid solute carbon), and having the heater-aimed electrode 12 buriedtherein; and an aluminum nitride layer 21 over the aluminum nitridelayer 20, comprising a sintered aluminum nitride body having a higheraverage concentration of free carbon, and having the electrostaticelectrode 14 buried therein.

Note that average concentrations of free carbon can be measured here byan infrared absorption method, by quantifying CO_(x) to be generatedupon heating a sintered aluminum nitride body at 900° C. Meanwhile, anaverage concentration of all carbon including free carbon and solidsolute carbon is a value obtained by the infrared absorption method byquantifying CO_(x) generated upon high-frequency heating or heating at1,350° C. (with addition of flux), and this quantification can beconducted by a method (JCRS105-1995) for analyzing an amount of C withinAlN, which meets the standards of the Ceramic Society of Japan.

Then, there is cut out a predetermined region 22 from the aluminumnitride substrate 18 as shown in FIG. 3(f), to obtain an aluminumnitride substrate 23 shown in FIG. 3(g).

Note that the production method of an aluminum nitride substrate is notlimited to the producing steps shown in FIG. 3, and the substrate may beproduced by producing steps shown in FIG. 4. In FIG. 4, like referencenumerals as used for the producing steps in FIG. 3 are used to denotecorresponding portions, and the explanation thereof is omitted.

Firstly, as shown in FIG. 4(a) and FIG. 4(b), prepared as startingmaterials are two types of aluminum nitride powders having differentcarbon concentrations of 600 ppm or lower.

Thereafter, the two types of aluminum nitride powders containing carbonpowders added thereto are used to load an aluminum nitride powder,another aluminum nitride powder including an electrostatic electrodeburied therein, and still another aluminum nitride powder including aheater-aimed electrode buried therein, into a mold (not shown) in athree-layered manner, and then these powders are molded. This yields athree-layered aluminum nitride mold 27 formed of an aluminum nitridelayer 24, an aluminum nitride layer 25 over it and having anelectrostatic electrode 14 buried therein, and an aluminum nitride layer26 over it and having a heater-aimed electrode 12 buried therein, asshown in FIG. 4(c) (molding step).

After molding, the aluminum nitride mold 27 is placed in a hot pressingapparatus 17 as shown in FIG. 4(d), and fired therein by hot pressing(firing step) into a three-layered sintered aluminum nitride body 28. Asshown FIG. 4(e), the sintered aluminum nitride body 28 is formed of: analuminum nitride layer 29 comprising a sintered aluminum nitride bodyhaving a lower average concentration of solid solute carbon; an aluminumnitride layer 30 over the aluminum nitride layer 29, comprising asintered aluminum nitride body having a higher average concentration ofsolid solute carbon, and having the electrostatic electrode 14 buriedtherein; and an aluminum nitride layer 31 over the aluminum nitridelayer 30 having the heater-aimed electrode 12 buried therein.

Then, there is cut out a predetermined region 32 from the sinteredaluminum nitride body 28 as shown in FIG. 4(f), to obtain an aluminumnitride substrate 33 shown in FIG. 4(g).

Note that although this embodiment has been described by the exampleswhere the aluminum nitride substrate 23, 33 having both the heater-aimedelectrode 12 and the electrostatic electrode 14 buried therein are usedas electrostatic chuck heaters, respectively, the invention is notlimited to an electrostatic chuck heater, and it is possible to use analuminum nitride substrate as: a ceramic heater having only aheater-aimed electrode 12 buried therein; an electrostatic chuck havingonly an electrostatic electrode 14 buried therein; or a high-frequencygenerating electrode device having a plasma-generating electrode buriedtherein. In the embodiment of the invention and in case ofconfigurations other than an electrostatic chuck, the work mountingportion “A”, and the region “B1” under an electrode and the region “B2”between electrode segments in the base layer “B”, refer to the followingregions, respectively.

Namely, FIG. 5A shows a longitudinal cross-sectional view of a singleelectrode type of electrostatic chuck 34 adopting the aluminum nitridesubstrate 1 according to another embodiment of the invention, and FIG.5B shows a radial cross-sectional view through an electrostaticelectrode 3 of the electrostatic chuck 34. Note that like referencenumerals as used for the electrostatic chuck heater shown in FIG. 1 areused to denote corresponding elements, and the explanation thereof isomitted. As shown in FIG. 5A and FIG. 5B, the electrostatic chuck 34exhibits a work mounting portion “A” which is a region over theelectrostatic electrode 3, and a base layer “B” having the electrostaticelectrode 3 buried therein, the base layer “B” including a region “B1”under the electrostatic electrode 3.

Further, FIG. 6A is a cross-sectional view of a dual electrode type ofelectrostatic chuck 35 adopting the aluminum nitride substrate 1according to still another embodiment of the invention, FIG. 6B is anaxial cross-sectional view of electrostatic electrodes 36 a, 36 b, andFIG. 6C is a radial cross-sectional view of the electrostatic electrodes36 a, 36 b. As shown in FIG. 6A through FIG. 6C, the electrostatic chuck35 has the electrostatic electrodes 36 a, 36 b buried therein, whichcomprise metal meshes separated from each other at central portionsthereof in a left-right symmetric manner. The substrate exhibits a workmounting portion “A” which is a region over the electrostatic electrodes36 a, 36 b, and a base layer “B” including a region “B1” under theelectrostatic electrodes 36 a, 36 b and a region “B2” between the rightand left electrostatic electrodes.

Further, FIG. 7A is a longitudinal cross-sectional view of a heater 37adopting the aluminum nitride substrate 1 according to yet anotherembodiment of the invention. The heater 37 has a metal foil 38 helicallyburied therein, as a heater-aimed electrode within the aluminum nitridesubstrate 1. FIG. 7B is an axial cross-sectional view through the metalfoil 38. The heater 37 shown in FIG. 7A and FIG. 7B exhibits a workmounting portion “A” which is a region over the metal foil 38, and abase layer “B” including a region “B1” under the metal foil 38 and aregion “B2” between adjacent segments of the metal foil 38.

FIG. 8A is a longitudinal cross-sectional view of a heater 39 in amodified electrode configuration provided by adopting the aluminumnitride substrate 1 according to a still further embodiment of theinvention. The heater 39 has a metal coil 40 as a heater-aimed electrodeburied within the aluminum nitride substrate 1. FIG. 8B is an axialcross-sectional view of the metal coil 40. In this case, the substrateexhibits a work mounting portion “A” comprising a region over the metalcoil 40, and a base layer “B” including a region “B1” under the metalcoil 40 and a region “B2” between adjacent segments of the metal coil40.

The invention will be further described by concrete Examples.

EXAMPLE 1

Fabricated in Example 1 was an aluminum nitride substrate by adoptingthe producing steps shown in FIG. 3.

Firstly, carbon was contained in an aluminum nitride powder to preparetwo kinds of aluminum nitride powders having different carbonconcentrations of 500 ppm or lower.

Next, the prepared aluminum nitride powders were used together with anHT coil as a heater-aimed electrode and an RF mesh as an electrostaticelectrode, and matured into a molded body.

Thereafter, the obtained molded body was fired under the conditionsshown in Table 1, to obtain a sintered aluminum nitride body having athickness of 25 mm. There was attained a carbon concentrationdistribution “E” by controlling the firing conditions as shown in Table1 to adjust a solid solute carbon amount and a free carbon amount. TABLE1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Com. Ex. 1 Com. Ex. 2 Starting Material Carbonconcentration (ppm) 500 550 300 300 300 250 in AlN powder Firingconditions Firing temperature (° C.) 1800 1800 1750 1750 1820 1850Holding time (hr) 1 1 2 2 1 40 Pressure (kg/cm²) 90 90 60 60 200 85Temperature elevation rate (° C./hr) 80 80 100 100 25 30 Degree ofvacuum (Pa) 13.33 13.33 6.66 6.66 13.33 6.66 Sintered Carbonconcentration distribution E F E F Uniform Uniform aluminium nitride insintered AlN body body Volume resistivity (W × cm)at RT,  1 × 10¹⁵/  1 ×10¹⁵/  5 × 10¹⁵/  5 × 10¹⁵/  8 × 10¹⁴/  6 × 10¹⁴/ of regions over/underelectrode 1 × 10¹⁵ 1 × 10¹⁵ 5 × 10¹⁵ 5 × 10¹⁵ 8 × 10¹⁴ 6 × 10¹⁴ Volumeresistivity (W × cm) at  5 × 10¹¹/  5 × 10¹⁰/  1 × 10¹²/  1 × 10¹¹/  1 ×10¹⁰/  1 × 10¹⁰/ 400° C., of regions over/under 5 × 10¹⁰ 5 × 10¹¹ 1 ×10¹¹ 1 × 10¹² 1 × 10¹⁰ 1 × 10¹⁰ electrode Thermal conductivity (W/mK) atRT, 160/150 150/160 155/145 145/155 145/145 140/140 of regionsover/under electrode Grain separation property 69 32 88 36 68 74 (numberof separated grains)

The aluminum nitride substrate having the carbon concentrationdistribution “E”, exhibited a work mounting portion “A” having anaverage concentration of free carbon higher than an averageconcentration of free carbon in a base layer “B” (region “B1” under theelectrode, and a region “B2” between segments of the electrode) in amanner that the work mounting portion “A” had the average concentrationof free carbon of 80 ppm, the region “B1” under the electrode had anaverage concentration of free carbon of 30 ppm, and the region “B2”between the electrode segments had an average concentration of freecarbon of 60 ppm. Further, the work mounting portion “A” had an averageconcentration of all carbon per unit volume, which was lower thanaverage concentrations of all carbon per unit volume of the region “B1”under the electrode and the region “B2” between electrode segments, in amanner that the work mounting portion “A” had the average concentrationof all carbon of 380 ppm, the region “B1” under the electrode had theaverage concentration of all carbon of 460 ppm, and the region “B2”between electrode segments had the average concentration of all carbonof 420 ppm. Moreover, the work mounting portion “A” had an averageconcentration of solid solute carbon, which was lower than averageconcentrations of solid solute carbon of the region “B1” under theelectrode and the region “B2” between the electrode segments, in amanner that the work mounting portion “A” had the average concentrationof solid solute carbon of 300 ppm, the region “B1” under the electrodehad the average concentration of solid solute carbon of 430 ppm, and theregion “B2” between the electrode segments had the average concentrationof solid solute carbon of 360 ppm.

EXAMPLE 2

In Example 2, there was produced a sintered aluminum nitride body byusing the production method shown in FIG. 3, identically to Example 1.Example 2 was different from Example 1, in that the carbon concentrationdistribution in the sintered aluminum nitride body of Example 1 waschanged. Namely, there was attained a carbon concentration distribution“F” by controlling the firing condition as shown in Table 1 to adjust asolid solute carbon amount and a free carbon amount.

The aluminum nitride substrate having the carbon concentrationdistribution “F”, exhibited a work mounting portion “A” having anaverage concentration of free carbon higher than average concentrationsof free carbon in a region “B1” under the electrode and a region “B2”between the electrode segments in a manner that the work mountingportion “A” had the average concentration of free carbon of 100 ppm, theregion “B1” under the electrode had an average concentration of freecarbon of 90 ppm, and the region “B2” between electrode segments had anaverage concentration of free carbon of 95 ppm. Further, the workmounting portion “A” had an average concentration of all carbon per unitvolume, which was higher than average concentrations of all carbon perunit volume of the region “B1” under the electrode and the region “B2”between electrode segments, in a manner that the work mounting portion“A” had the average concentration of all carbon of 550 ppm, the region“B1” under the electrode had the average concentration of all carbon of500 ppm, and the region “B2” between electrode segments had the averageconcentration of all carbon of 520 ppm. Moreover, the work mountingportion “A” had an average concentration of solid solute carbon, whichwas higher than average concentrations of solid solute carbon of theregion “B1” under the electrode and the region “B2” between theelectrode segments, in a manner that the work mounting portion “A” hadthe average concentration of solidly dissolved all carbon of 450 ppm,the region “B1” under the electrode had the average concentration ofsolidly dissolved all carbon of 410 ppm, and the region “B2” between theelectrode segments had the average concentration of solidly dissolvedall carbon of 425 ppm.

EXAMPLE 3

In Example 3, the firing condition of Example 1 was changed whileadopting aluminum nitride powders having carbon concentrations of 300ppm or lower to fabricate a sintered aluminum nitride body, to obtain analuminum nitride substrate having the carbon concentration distribution“E”. Concretely, the work mounting portion “A” had an averageconcentration of free carbon of 120 ppm, the region “B1” under theelectrode had an average concentration of free carbon of 40 ppm, and theregion “B2” between the electrode segments had an average concentrationof free carbon of 70 ppm. Further, the work mounting portion “A” had anaverage concentration of all carbon of 185 ppm, the region “B1” underthe electrode had an average concentration of all carbon of 280 ppm, andthe region “B2” between the electrode segments had an averageconcentration of all carbon of 230 ppm. Moreover, the work mountingportion “A” had an average concentration of solid solute carbon of 65ppm, the region “B1” under the electrode had an average concentration ofsolid solute carbon of 240 ppm, and the region “B2” between theelectrode segments had an average concentration of solid solute carbonof 160 ppm.

EXAMPLE 4

In Example 4, there was adopted the same firing condition as that ofExample 1, while adopting aluminum nitride powders having carbonconcentrations of 300 ppm or lower to fabricate a sintered aluminumnitride body, to obtain an aluminum nitride substrate having the carbonconcentration distribution “F”. Concretely, the work mounting portion“A” had an average concentration of free carbon of 110 ppm, the region“B1” under the electrode had an average concentration of free carbon of45 ppm, and the region “B2” between the electrode segments had anaverage concentration of free carbon of 75 ppm. Further, the workmounting portion “A” had an average concentration of all carbon of 295ppm, the region “B1” under the electrode had an average concentration ofall carbon of 180 ppm, and the region “B2” between the electrodesegments had an average concentration of all carbon of 235 ppm.Moreover, the work mounting portion “A” had an average concentration ofsolid solute carbon of 185 ppm, the region “B1” under the electrode hadan average concentration of solid solute carbon of 135 ppm, and theregion “B2” between the electrode segments had an average concentrationof solid solute carbon of 160 ppm.

COMPARATIVE EXAMPLE 1

In Comparative Example 1, there was adopted a different firingcondition, while adopting an aluminum nitride powder having a carbonconcentration of 300 ppm or lower to fabricate a sintered aluminumnitride body, as an aluminum nitride substrate having a uniform carbondistribution.

COMPARATIVE EXAMPLE 2

In Comparative Example 2, there was adopted another different firingcondition, while adopting an aluminum nitride powder having a carbonconcentration of 250 ppm or lower to fabricate a sintered aluminumnitride body, as an aluminum nitride substrate having a uniform carbondistribution.

The aluminum nitride substrates obtained in the Example 1 throughExample 4, and Comparative Example 1 and Comparative Example 2 were usedfor evaluation of volume resistivity, thermal conductivity, and grainseparation property thereof.

The volume resistance was measured over a range from a room temperature(38° C.) to a high temperature (400° C.) under vacuum in conformity toJIS C2141, by using a super-insulation resistance/minute electriccurrent meter (TR8601: ADVANTEST). Used for measurement of the volumeresistivity was a test piece in a shape of φ50 mm×1 mm or 50 mm×59 mm×1mm, and electrodes were formed of silver such that a main electrodediameter was 20 mm, a guard electrode inner diameter was 30 mm, a guardelectrode outer diameter was 40 mm, and an application electrodediameter was 40 mm. There was read each electric current value after alapse of one minute from application of a voltage of 500V, to calculatea volume resistivity.

The thermal conductivity was measured by a laser flash method by using athermal constant measuring apparatus (LF/TCM-FA8510B, manufactured byRigaku Corporation).

The grain separation property was evaluated as follows. Firstly, therewas formed an electrode on a mirror-finished side of each disk-like testpiece, while placing a silicon wafer having a diameter of 150 mm on aheater. Further, the disk-like test piece was stackedly placed on thesilicon wafer such that the mirror-finished side of the test piece wascontacted with the silicon wafer. Then, the heater was energized tostabilize the temperature of the disk-like test piece at 400° C.,followed by application of a voltage of 500V between the silicon waferand the disk-like test piece so that the silicon wafer and test piecewere attracted to each other for one minute. Thereafter, cooling wasconducted, and a portion of 10 mm 2 to 50 mm² of the disk-like testpiece was used for observation by an electron microscope. There wascounted the number of separated aluminum nitride grains by means of theelectron microscope, and calculated as a number of grains perpredetermined area (4,416 mm²) having a diameter of 75 mm, asexemplified by the result shown in Table 1.

As seen from the aluminum nitride substrate according to each ofComparative Example 1 and Comparative Example 2 in Table 1, it waspossible to enhance soaking ability of the aluminum nitride substrateitself, by uniformalizing the average concentration of solid solutecarbon of the corresponding sintered aluminum nitride body. Contrary, ineach of the aluminum nitride substrates according to Example 1 throughExample 4, the average concentration of solid solute carbon or freecarbon in the corresponding sintered aluminum nitride body was madenon-uniform, thereby enabling assurance of a quality commensurate withrequired properties. Particularly, it became apparent that, in each ofExample 2 and Example 4, since the average concentrations of solidsolute carbon in the region “B1” under the electrode and the region “B2”between the electrode segments were made lower than that of the workmounting portion “A” and the average concentration of free carbon on theelectrode surface was increased, the thermal conductivity of thealuminum nitride substrate surface was enhanced so that the soakingability of the aluminum nitride substrate was improved. Further, in eachof the aluminum nitride substrates of Example 1 and Example 3, there wasrestricted decrease of a volume resistance in a high temperature rangeof about 400° C. between the electrostatic electrode and theheater-aimed electrode, or between the RF terminal hole and the HTterminal holes, thereby enabling decrease of leakage current.

On the other hand, it became apparent that, in each of Example 1 andExample 3, since the average concentrations of solid solute carbon inthe region “B1” under the electrode and the region “B2” between theelectrode segments were made higher than that of the work mountingportion “A” and the average concentration of free carbon at grainboundary surfaces was decreased, thereby enabling decrease of grainseparation in number at a surface of the aluminum nitride substrate.Further, in view of a tendency that an application of pressure onto analuminum nitride substrate surface is prone to grain separation, thealuminum nitride substrates according to Example 1 and Example 3, inwhich the average concentrations of solid solute carbon in the region“B1” under the electrode and the region “B2” between the electrodesegments are higher than that of the work mounting portion “A”, are usedparticularly in a situation where each aluminum nitride substrate is tobe mirror-finished, thereby enabling decrease of grain separation innumber and assurance of a quality commensurate with required properties.

The contents of Japanese Patent Application No. 2004-096032, filed tothe Japanese Patent Office on Mar. 29, 2004, are incorporated herein byreference.

Although the invention has been described based on the embodiments, theinvention is not limited thereto, and various modifications may be madethereto without departing from the spirit or scope of the invention.

1. An aluminum nitride substrate comprising: a planer sintered aluminum nitride body containing carbon including free carbon; and a one-dimensional or two-dimensional electrode buried in the sintered aluminum nitride body, wherein the sintered aluminum nitride body comprises: a work mounting portion for placing a work thereon; and a base layer having the electrode buried therein; an average concentration of free carbon in the work mounting portion being different from an average concentration of free carbon in the base layer.
 2. The aluminum nitride substrate as claimed in claim 1, wherein the average concentration of free carbon of the work mounting portion is higher than the average concentration of free carbon of the base layer, and wherein an average concentration of all carbon per unit volume of the work mounting portion is lower than an average concentration of all carbon per unit volume of the base layer.
 3. The aluminum nitride substrate as claimed in claim 1, wherein the average concentration of free carbon of the work mounting portion is higher than the average concentration of free carbon of the base layer, and wherein an average concentration of all carbon per unit volume of the work mounting portion is higher than an average concentration of all carbon per unit volume of the base layer.
 4. The aluminum nitride substrate as claimed in claim 1, wherein the average concentrations of free carbon are 500 ppm or lower.
 5. The aluminum nitride substrate as claimed in claim 2, wherein the average concentrations of all carbon are 600 ppm or lower.
 6. The aluminum nitride substrate as claimed in claim 1, wherein the aluminum nitride substrate is used as a ceramic substrate for a semiconductor production and/or inspection apparatus.
 7. A production method of an aluminum nitride substrate, comprising: using at least two types of aluminum nitride powders having different carbon concentrations of 600 ppm or lower, respectively, and burying an electrode within the powders and molding them into an aluminum nitride mold in a structure comprising at least two layers having different carbon concentrations; and firing the obtained aluminum nitride mold at 1,700° C. to 1,950° C., into an aluminum nitride substrate comprising a work mounting portion for placing a work thereon, and a base layer having the electrode buried therein, an average concentration of free carbon in the work mounting portion being different from an average concentration of free carbon in the base layer.
 8. The production method of an aluminum nitride substrate as claimed in claim 7, wherein the aluminum nitride mold is fired under conditions of a pressure of 50 kg/cm² to 250 kg/cm² a temperature elevation rate of 10° C./hr to 120° C./hr, and a degree of vacuum of 1.3×10⁻¹ Pa to 133.3 Pa. 